1. Field of the Invention
The present invention relates generally to a distributed amplifier, more particularly, to a distributed amplifier as a monolithic microwave integrated circuit (MMIC).
2. Description of the Related Art
The distributed amplifier has been employed at a stage before electric-to-photo conversion or after photoelectric conversion since it has wideband characteristics.
FIG. 7 shows a prior art typical distributed amplifier.
A terminating circuit 29 of an output transmission line 20 having elements 21 to 28 consists of a terminating resistor 291 almost equal to the characteristic impedance of the transmission line 20 and a capacitor 292 for AC grounding, serially connected to each other. The capacitor 292 can reduce power consumption caused by applying a drain bias voltage VDD to the resistor 291 if the capacitor 292 is not connected.
An output voltage signal Vout=0 when an input voltage signal Vin=0. In this state, a DC gate bias voltage VGG and a DC drain bias voltage VDD are respectively applied to the gate and drain of each of FETs 31 to 34 to force a DC bias current to flow through each of the FETs 31 to 34.
When the input voltage signal Vin is superimposed on the bias voltage VGG, the signal Vin propagates along an input transmission line 10 and a part thereof is applied to the gates of FETs 31 to 34. In the FET 31 for example, a signal component (i1+i2)) is superimposed on the bias current, wherein i1 and i2 are currents flowing through the output transmission line 20 to the terminating circuit 29 and the output OUT, respectively. Currents flowing from the amplifying FETs 31 to 34 to the output OUT are simply summed at the output OUT since line lengths from the input IN to the output OUT through the respective FETs 31 to 34 are the same as each other and in turn the respective currents therefrom have the same phase at the output OUT.
In order to make a frequency characteristic of the gain wider in bandwidth, it is necessary to employ FETs 31 to 34 having smaller gate capacitances. However, as the gate capacitances are smaller, the gains of the FETs 31 to 34 become lower.
In order to solve this problem, employed is a distributed amplifier configured such that, as shown in FIG. 8, capacitors 51 to 54 are connected between the gates of respective FETs 31 to 34 and the input transmission line 10 and thereby, a combined capacitance of each gate capacitance and each capacitor is reduced. In this configuration, the gate bias voltage VGG is applied to the gates of FETs 31 to 34 through resistors 41 to 44, respectively.
In both of the distributed amplifiers of FIGS. 7 and 8, since the impedance of the capacitor 292 can be neglected in regard to the high frequency components of the input voltage Vin, the impedances of the terminating circuit side and the output OUT side viewed from the drain of the FET 31 are almost equal to the characteristic impedance, leading to the relation of i1=i2. This applies to each case of the FETs 32 to 34 in similar manner. However, since the capacitance of the capacitor 292 cannot be neglected in regard to the low frequency components of the input voltage signal Vin, the relation of i1 less than i2 holds. This again applies to each case of the FETs 32 to 34 in similar manner. For this reason, as shown in FIG. 4, the gain of the distributed amplifier in a low frequency band is higher than that in a high frequency band where the gain stays flat, and it tends to increase as the frequency is lower in the low frequency band.
If the capacitance 292 is omitted in order to prevent the increase in the gain in the low frequency band, power consumed in the distributed amplifier is increased by the drain bias voltage VDD applied to the resistor 291.
Accordingly, it is an object of the present invention to provide a distributed amplifier capable of improving a flatness of its gain over a low frequency band in a case where a capacitor for ac-grounding is employed in a terminating circuit of an output transmission line.
In one aspect of the present invention, there is provided a distributed amplifier comprising a plurality of series-connected circuits, provided for respective amplifying transistor, each including a capacitor and a resistor connected in series to each other between the control input of the corresponding transistor and a reference potential, each having impedance lower than the input impedance of the transistor in a frequency band lower than a frequency, but higher than this input impedance in a frequency band higher than this frequency.
A current flowing through each series-connected circuit can be neglected in the high frequency band, but cannot be neglected in the low frequency band. As a frequency is lower, an input signal to the transistor decreases and the output signal thereof decreases in the low frequency band.
On the other hand, as a frequency is lower, the impedance of a terminating circuit including a capacitor and a resistor connected in series increases in the low frequency band, thereby a current signal flowing to the output side of an output transmission line from the transistor is larger than a current signal flowing to the terminating circuit located at the opposite side thereof.
Hence, the amplitude of the current signal flowing to the output side can be flattened in magnitude over the low frequency band. That is, the flatness of the gain over the low frequency band can be improved.